I got the reflash to work, but unfortunately it didn't fix the problem. It still fails the loopback test, and uploading a sketch won't work either, do you have some more things I could try or should I write this one off?
Even if the Loopback failed, since you were able to reflash the chip with no errors, the Atmega16u2 should be fine. Now, the focus can turn to the bootloader, but we do need an ISP (Second Arduino or AVRISP).
For grins, try this:
Hold down the reset button and unplug the USB cable.
Select a sketch and upload to the Uno (Reset still held down).
When any activity occurs on the Rx/Tx LEDs, release the button.